Radiosurgery and radiotherapy systems are radiation treatment systems that use external radiation beams to treat pathological anatomies (e.g., tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering a prescribed dose of radiation (e.g., X-rays or gamma rays) to the pathological anatomy while minimizing radiation exposure to surrounding tissue and critical anatomical structures (e.g., the spinal chord). Both radiosurgery and radiotherapy are designed to necrotize the pathological anatomy while sparing healthy tissue and the critical structures. Radiotherapy is characterized by a low radiation dose per treatment, and many treatments (e.g., 30 to 45 days of treatment). Radiosurgery is characterized by a relatively high radiation dose in one, or at most a few, treatments.
In both radiotherapy and radiosurgery, the radiation dose is delivered to the site of the pathological anatomy from multiple angles. As the angle of each radiation beam is different, each beam can intersect a target region occupied by the pathological anatomy, while passing through different regions of healthy tissue on its way to and from the target region. As a result, the cumulative radiation dose in the target region is high and the average radiation dose to healthy tissue and critical structures is low. Radiotherapy and radiosurgery treatment systems can be classified as frame-based or image-guided.
In frame-based radiosurgery and radiotherapy, a rigid and invasive frame is fixed to the patient to immobilize the patient throughout a diagnostic imaging and treatment planning phase, and a subsequent treatment delivery phase. The frame is fixed on the patient during the entire process. Image-guided radiosurgery and radiotherapy (IGR) eliminate the need for invasive frame fixation by tracking and correcting for patient movement during treatment.
Image-guided radiotherapy and radiosurgery systems may be classified as gantry-based or robotic-based. In gantry-based systems, the radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. In some gantry-based systems, known as intensity modulated radiation therapy (IMRT) systems, the cross-section of the beam is shaped to conform the beam to the pathological anatomy under treatment. In robotic-based systems, the radiation source is not constrained to a single plane of rotation.
In some image-guided systems, patient tracking during treatment is accomplished by registering 2-D in-treatment X-ray images of the patient (indicating where the patient is) to 2-D reference projections of one or more pre-treatment 3-D volume studies of the patient (indicating where the patient should be to match the treatment plan), and changing the position of the patient or the radiation source to correct for differences between the two sets of images. The pre-treatment 3-D volume studies may be computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, positron emission tomography (PET) scans or the like.
The reference projections (reference images), known as digitally reconstructed radiographs (DRRs), are generated using ray-tracing algorithms that replicate the known geometry of the in-treatment X-ray imaging system to produce images that have the same scale and orientation as the in-treatment X-ray images. Typically, the in-treatment X-ray system images the patient using two X-ray sources and two X-ray cameras subtending large angles (e.g., 90 degrees) at the patient. This approach maximizes the sensitivity of the individual in-treatment X-ray images to patient movement, but it can produce two very dissimilar X-ray images as illustrated in FIG. 1. In FIG. 1, an anatomical feature (e.g., a bone) is imaged with two X-ray sources and two X-ray cameras separated by 90 degrees. In one camera, the length and width of the bone is imaged, while in the other camera, the cross-section of the bone is imaged. The two X-ray images are very dissimilar, requiring a separate DRR for registration with each X-ray image before the location of the patient can be determined and matched to the pre-treatment plan.